The invention relates to improvements in the manufacture of composites of crystalline aluminosilicate zeolite and amorphous alumina-silica in the form of mechanically strong monolithic bodies. The invention relates particularly to an improvement in those processes for producing such composites which involve the in situ synthesis of a zeolite alumino-silicate, such as faujasite, ZSM-5 or mordenite, by reaction of a solution of a base with calcined clay contained in preformed self-supporting monolithic bodies.
Zeolitic molecular sieves are used in a wide variety of catalytic and adsorptive applications. For example, sieves such as faujasites and ZSM-5 are well-known constituents of hydrocarbon conversion catalysts. Other synthetic zeolites such as mordenite are useful as catalysts for the reduction of nitrogen oxides with ammonia. The zeolites are normally synthesized as finely divided high purity crystals. For most purposes the crystals must be bonded with a suitable matrix material such as a silica-alumina gel, clay or mixture thereof, to form particles having good attrition resistance, high heat capacity and thermal conductivity. The choice of a binder for a zeolitic molecular sieve is limited by the fact that the binder must be thermally stable and provide access of gases or liquids to the zeolite crystals in the composite particles.
Zeolitic molecular sieve catalyst or catalyst support particles are supplied in the form of small microspheres, typically particles having an average size of about 60 microns, when they are to be used in fluidized bed processing such as fluidized bed catalytic cracking of gas-oil feedstocks. Generally the particles are in the form of cylinders or spheres that are 1/16 inch or larger when they are to be used in fixed bed processes such as the hydrocracking or hydrotreating of resid hydrocarbons. On the other hand, gas phase reactions carried out at high space velocity and liquid phase reactions of heavy oils are often diffusion limited, i.e., only the outer portion of the catalyst particles is utilized. Catalysts for such reactions are desirable in the form of thin-walled honeycombs. Irrespective of the specific form or shape of the catalyst or absorbent bodies, it is generally desirable to provide the structures in the form of rigid, attrition-resistant bodies in which zeolite crystals are uniformly disseminated in a porous heat-stable matrix.
The synthesis of zeolites from calcined clays, especially kaolin clay, is known. For example, it is well known that metakaolin (kaolin clay calcined at a temperature of about 1200.degree. to 1500.degree. F.) will react with sodium hydroxide solution to produce sodium zeolite A. It is also known that when kaolin is calcined under more severe conditions, sufficient to undergo the characteristic exothermic reaction, for example 1700.degree. to 2000.degree. F., the calcined clay will react with sodium hydroxide solution, small amounts of metakaolin preferably being present, to synthesize faujasite-type zeolites. Reference is made to the following commonly assigned patents of Haden et al: U.S. Pat. Nos. 3,335,098 and 3,338,672. As an offshoot of these discoveries processes were invented that resulted in shaped bodies that were composites of a mixture of crystals of faujasite-type zeolites and porous silica-alumina matrix. The composites were synthesized directly in the form of shaped particles, in particular fluidizable microspheres, from preforms composed of kaolin clay calcined to undergo the exotherm. This was accomplished by immersing the preforms (microspheres of kaolin clay calcined at high temperature) in a solution of sodium hydroxide to form a slurry, aging the slurry, typically for 4-8 hours at 100.degree. F. and then heating to crystallize the zeolite within the preforms. Silica originally in the microspheres was leached or extracted during the reaction, producing a sodium silicate mother liquor and leaving a porous matrix in the zeolitized microspheres. Because the composite bodies were zeolitized directly without a separate binding step to composite zeolite and binder, the processing has become known as the "in situ" process. Reference is made to the following commonly assigned patents of Haden et al: U.S. Pat. Nos. 3,391,994, 3,433,587, 3,503,900 3,506,594, 3,647,718 and 3,663,165 and 3,932,268.
It is now known that the in situ technology can be utilized to convert bodies of kaolin clay calcined to undergo the exotherm into composite bodies in which the zeolitic component is other than a member of the faujasite family. For example, the crystalline aluminosilicate component can be synthetic crystalline mordenite or ZSM-5 type zeolites. Reference is made to U.S. Pat. No. 4,091,007 to Dwyer et al and to our copending U.S. application Ser. No. 864,731 dated Dec. 27, 1977 and now abandoned, the entire disclosures of which are incorporated herein by cross-reference. Further, it has also been discovered that calcined clay-containing precursor bodies and ultimate zeolitized products can take forms other than fluidizable microspheres. For example, the bodies may be cylindrical pellets, berl saddles or they may even have complex intricate shapes such as multi-channeled structures or honeycombs. Reference is made to our copending application Ser. No. 856,658 filed Dec. 2, 1977 and now abandoned, the entire disclosure of which is also incorporated herein by cross-reference and to U.S. Pat. No. 4,091,007 (supra).
Irrespective of the zeolite to be synthesized by in situ reaction between preformed bodies composed of kaolin clay calcined to undergo the exotherm and basic solutions, the source of clay and the calcining conditions have a significant effect on the process. Calcination of hydrated kaolin clay results in dramatic changes in the reactivity of the clay towards both bases and acids. Especially when the clay is calcined at temperature sufficiently high to undergo the exotherm, the reactivity of the calcined clay is remarkably sensitive to the source of the hydrated clay employed as a starting material and reactivity is also highly sensitive to calcination conditions. For reasons not presently understood even high purity kaolin clays from different sources frequently react differently towards acids and bases when calcined under essentially the same conditions and in the same equipment. The difference in reactivity towards basic solutions is reflected in rate of reaction and/or by the composition and quantity of the crystalline zeolitic aluminosilicate present in the zeolitized bodies. This can present quality control problems of considerable magnitude. For example, in the manufacture of a faujasite cracking catalyst it is generally desirable to synthesize a faujasite component having a consistently high SiO.sub.2 /Al.sub.2 O.sub.3 and in a consistent quantity. In practice this means that when the manufacturer of a cracking catalyst utilizing the in situ approach employs a new source of clay, an undesirably low zeolite content may be found in the zeolitized bodies or the SiO.sub.2 Al.sub.2 O.sub.3 of the zeolite component may be less than desired. This may also occur when there are fluctuations in the operation of the calciner.
Sensitivity of the in situ processing to variations in clay and calcination conditions is minimized to a certain extent by including a small amount of the form of calcined clay generally referred to as "metakaolin" in the reaction mixture. As noted in several of the patents cited above, metakaolin is prepared under conditions that are relatively mild compared to those employed when the clay undergoes the exotherm. While the addition of metakaolin has the effect of "smoothing out" the process, it does not assure that the desired zeolite content and zeolite composition will be achieved under any conditions, much less at an acceptable production rate, irrespective of the clay source and the conditions employed when the green clay bodies are calcined to undergo the exotherm.
Practice of present invention incorporates the feature of carrying out the reaction between the shaped precursor bodies of calcined clay and aqueous reaction liquid in the presence of added nucleation centers, i.e., a dilute aluminosilicate solution, generally of a colloidal nature, and chemically akin to the crystalline zeolite that is to be synthesized. Such nucleation centers, frequently referred to as "seeds" or crystallization "directors", have been used in a wide variety of crystallization operations. These have included processes for preparing zeolitic aluminosilicates of the synthetic faujasite type. In accordance with the teachings of U.S. Pat. No. 3,808,326 nucleation centers are used in the synthesis of so-called zeolite Y (U.S. Pat. No. 3,130,007) from active SiO.sub.2 /Al.sub.2 O.sub.3 gels. As a result, inception time and reaction rate are reduced. Solutions of nucleation centers are also used in the process of U.S. Pat. No. 3,671,191 but they are employed with a mineral acid to prevent silica solubilization by excess caustic, the acid thus allowing growth of higher SiO.sub.2 /Al.sub.2 O.sub.3 faujasite. Without the nucleation centers, too long a reaction time would result. Crystalline nucleation centers are utilized in practice of the invention of U.S. Pat. No. 3,574,538. This is in contrast to the amorphous nucleation centers employed in the processes of the above patents. U.S. Pat. No. 3,547,538 teaches that heat accelerates maturation of nucleation centers. Again, the nucleation centers are employed simply to increase reaction rate of faujasite-type zeolites. U.S. Pat. No. 3,492,090 also concerns a seeded reaction for production of synthetic crystalline zeolites of the faujasite type. The feature of this patent is that after addition of nucleation centers and silica-alumina gel the mixture is deliquored and the solid cake is reacted at 200.degree. F. Reduction in material handling is cited as the advantage. In U.S. Pat. No. 3,777,006 metakaolin is mixed with sodium silicate to correct for differences in SiO.sub.2 /Al.sub.2 O.sub.3 between the clay and the desired crystalline zeolite Y reaction product. The mixture is formed into particles, dried for 16-24 hours to impart hardness, and then reacted with caustic and a solution of nucleation centers to produce particles composed essentially or substantially so, of zeolite Y. British Pat. Nos. 1,271,450 and 1,342,977 deal with processes generally similar to the one described in U.S. Pat. No. 3,777,006. In the processes of the British patents, the metakaolin and sodium silicate (or silica-alumina gel) are formed into small fluidizable spheres by spray drying slurries of the mixtures. In the former patent (U.S. Pat. No. 1,271,450), the zeolitic nucleation centers are included in the slurry feed to the spray drier and the spray dried product is reacted with caustic to form zeolite. In the latter patent (U.S. Pat. No. 1,342,977), the seeds are added after spray drying to the caustic reaction slurry. The method allegedly allows formation of harder particles, since the particles can be calcined after spray drying without fear of affecting seed integrity.